1. Field of the Invention
The present invention generally relates to a method for moving a material removal tool over a substrate surface. More particularly, the present invention relates to a method of moving a small material removal tool, such as a plasma assisted chemical etching tool or mechanical polishing tool, so that material is removed from the surface of the substrate by moving the tool within predetermined spatial intervals with low tool acceleration within those intervals. The present invention provides a method for making an object having a predetermined final thickness profile from a substrate such as a silicon-on-insulator (SOI) wafer.
2. Description of the Prior Art
Substrate materials with thin solid layers, such as SOI wafers, and films are used extensively in the fabrication of electronic, optical, magnetic, superconducting and other important technological devices. For various reasons, such substrates are often subjected to figuring or thinning processes to remove some of the material from the surface. A method of computer controlled corrections by means of a plasma assisted chemical etching material removal tool to modify film thickness profiles of SOI wafers is disclosed in a U.S. patent application, Ser. No. 07/807,544, filed on Dec. 13, 1991, entitled "Method to Determine Tool Paths for Thinning and Correcting Errors in Thickness Profiles of Films", now U.S. Pat. No. 5,291,415, and is assigned to the assignee hereof. The disclosure therein is directed to a method for the development of a removal tool constant velocity versus removal tool position map from a removal tool dwell-time versus removal tool position map. The removal tool dwell-time versus position map provides information to relative motion hardware, such as a 5-axis position controller and stage, relating to the time the removal tool must spend over each of a plurality of predefined areas or spatial intervals of the surface of a wafer to remove a predetermined amount of material within each interval to achieve a desired thickness profile. In general, 5-axis position control hardware is required by most any method for material removal, including the constant velocity method, to follow an arbitrarily shaped surface. For applications in which the substrate to be corrected is nearly flat, such as thinning the silicon layer on an SOI structure or flattening silicon wafers, 2-axis motion is all that is needed. Common 2-axis motion configurations may be linear scan and step, "X-Y", and substrate table rotation with radial translation, "R-.theta.". While the following discussion refers to X-Y motion, it applies equally well to other motion configurations such that it relates to executing the dwell-time map regardless of the configuration of the position control hardware.
Under the constant velocity method in general, the removal tool is moved over each predefined spatial interval at a constant velocity such that the time the tool spends over that predefined spatial interval corresponds to a dwell-time calculated for that interval. However, the constant velocity method for moving a material removal tool has drawbacks in that the stage must be able to provide very rapid acceleration or deceleration between each adjacent spatial interval in order to provide nearly constant velocity within each spatial interval. In other words, the changes in the constant velocity between spatial intervals must occur in a very short period of time so that the velocity of the removal tool is nearly constant within substantially all of the spatial interval. Unfortunately, the maximum acceleration or deceleration of the stage ultimately limits the ability to accurately alter the SOI to a desired thickness profile under the constant velocity method. The acceleration of the tool with respect to the substrate is a function of the average tool velocity and the change in dwell-time between adjacent spatial intervals. In practice, a production application requiring rapid correction of parts (e.g., thickness of silicon in a SOI structure, flattening of silicon wafers) would require a high tool velocity and, consequently, very high accelerations if the dwell-time map is executed by the constant velocity method. Furthermore, such accelerations may not be practical with ordinary motion control hardware because the demand of rapid acceleration and deceleration may adversely effect its lifetime, reliability and cost. In some cases the demands may be so great as to require very expensive, specialized, massive control hardware.
Thus, the present invention is directed toward a method to overcome the shortcomings of material removal from the surface of a substrate that are associated with the constant velocity method for moving a material removal tool by providing a method that allows for a minimized acceleration of the stage within a spatial interval that satisfies the dwell-time requirements for that spatial interval.